In recent years, to improve automotive fuel consumption and widen an automotive indoor space, noise reduction, downsizing, and weight saving of automotive parts are advancing. Owing to the production of noise-reduced automotive parts in recent years, an engine room is sealed to a higher extent. Thus, grease for a rolling bearing for use in automotive electric auxiliary machines is demanded to have high-temperature durability. To compensate the downsizing-caused reduction in the outputs of the automotive electric auxiliary machines by rotating the electric auxiliary machines at high speeds, the rolling bearing is used at high speeds and under high loads. Because the use environment for the rolling bearing is becoming increasingly severe, there is a report on the occurrence of a peeling phenomenon accompanied by the structural change of rolling surfaces of the rolling bearing into white, namely, on the occurrence of the hydrogen brittleness-caused peeling phenomenon. Under these circumstances, the grease is demanded to take countermeasures for preventing the occurrence of the hydrogen brittleness-caused peeling phenomenon. In cold districts such as Russia, North America, abnormal noises are generated when an engine is started. The generation of so-called abnormal noises in cold environment has become a problem. Thus, the grease is also demanded to have further improvement in its low-temperature property. In addition, the grease may be subjected to rainwater while automobiles are traveling. Therefore, the grease is also demanded to be rust-preventive. In this situation, the grease is demanded to satisfy all of the high-temperature durability, the peeling resistance, the low-temperature property, and the rust-preventive property.
As grease for the rolling bearing for use in the automotive electric auxiliary machines, grease using base oil such as synthetic hydrocarbon oil, alkyl diphenyl ether oil or ester synthetic oil is most popular. The grease containing the synthetic hydrocarbon oil as its main component is short of its high-temperature durability. The grease containing the alkyl diphenyl ether oil as its main component is short of its low-temperature property. It may be difficult for the grease using the ester synthetic oil as its base oil to achieve its heat resistance and low-temperature property.
As grease having excellent high-temperature durability, diurea grease using the ester synthetic oil is known (patent document 1). As grease having excellent low-temperature property, diurea grease using mixed oil of trimethylolpropane or pentaerythritol ester synthetic oil and the synthetic hydrocarbon oil is known (patent document 2).
As grease having excellent resistance to the occurrence of the hydrogen brittleness-caused peeling (hereinafter referred to as peeling resistance) phenomenon, diurea grease containing molybdates is known (patent document 3).
As grease having excellent rust-preventive property and peeling resistance, diurea grease containing zinc naphthenate and alkenyl succinic acid half ester is known (patent document 4).
Although the grease described in the patent documents 1 through 4 is excellent in anyone or two properties of the high-temperature durability, the low-temperature property, the peeling resistance, and the rust-preventive property, any of the grease is not capable of satisfying all of these properties. As the automotive parts are becoming more silent, smaller, and more lightweight, grease compositions for bearings for use in the automotive parts and particularly grease compositions for the outer ring rotation type rolling bearing are required to satisfy not only one of the above-described properties, but all of the properties.